Electric heating system



G. NIGRA ELECTRIC HEATING SYSTEM Sept. 17, 1940.

Filed Dec. 6, 1938 2 Sheets-Sheet l ep 1 0- G. NIGRA ELECTRIC HEATING SYSTEM 2 Sheets-Sheet 2 Filed Dec. 6, 1938 Fig.2

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vvrrrr w'IV A Patented Sept. 17, 1940 UNITED STATES PATENT OFFICE ELECTRIC HEATING SYSTEM Giuseppe Nigra, Turin, Italy, assignor to Socleta. Anonima.Manitattura Ceramico Pozzi, Turin,

Electric heating units are presently constituted by resistances of metal or various composition which are heated by the flow of current or by arcs formed between hot' relatively low-voltage electrodes generally carbon electrodes.

These known systems are in some cases subject to inconveniences consisting in that the metallic resistance cannot practically exceed a temperature of 1100 C; other resistances generally constituted by carbides or carbons of various forms may exceed said temperature but are of short life and difiicult to control.

The system of the usual low-voltage arcs be tween carbon electrodes shows the inconvenience of a rapid wearing out of the electrodes, an exceedingly high temperature and infection by the carbon of the surrounding air.

The system according to this invention is particularly suitable for high temperatures that is to say above 1200 C. and removes the inconveniences of the known systems using such high temperatures.

The system according to this invention is con: i

i stituted by two or a plurality of electrodes of metal or other conducting material cooled by means of a cooling fluid generally circulating in the interior thereof; said electrodes are enclosed in a space of suitable small size in order to ensure a regular working and are brought to a potential difference of some thousand volts, so that the arc strikes spontaneously between them and being fed through suitable electric devices keeps continuously and steadily even when it is blown towards an outlet under the action of a suitable fluid introduced in the space, in such manner as to transfer, by means of said fluid which becomes heated in contact with the arc, theheat generated by the arc to the region where it is utilised.

Various means are resorted to for preventing harmful reactions of chemical nature that might be generated by the heat transferring fluid subjected to the high temperature of the arc.

The heating system according to this invention is more particularly suited for heating pottery baking kilns since a clean environment and an even distributed heat are obtained. Thus for instance for a small baking kiln heated to 1400 C. a 40 kw. electric heater is sufficient. Obviously said heater may be applied to any type or ordinary kilns in the same manner as a heavy oil heater.

The heating system according to this invention is substantially constituted by two parts, that is to say a proper electric heater and a feeding unit of high voltage power for striking the arc, of pressure air for blowing the arc and of cooling Water for the electrodes.

Means for producing a high pressure are required for feeding the air, as the air speed must be very high; said pressure may be obtained by means of ordinary centrifugal fans.

The ordinary pressure of drinkable water supply are sufiicient for the water feed.

In view of the high voltage used the electric power'feecling unit must be isolated and protected against any contact with high voltage parts. Generally a single phase booster transformer and a stabilising flywheehimpedance 5 having the same size as those used for the ordinary low voltage arcs are provided for this feeding.

The annexed drawing shows by way of example an embodiment of the heating system according to this invention. v

Fig. 1 is a horizontal axial section thereof,

Fig. 2 is an elevation partially in section of a small mufile kiln for baking pottery;

Fig. 3 is a cross vertical section thereof.

Fig. 4 shows in elevation, partially sectioned,

a crucible furnace.

Referring to Fig. 1, the heating system comprises a conical chamber A of high resistance refractory material fixed by means of two 001- lars in one piece therewith to the casing C of sheet iron enclosing the unit. Said conical chamber is provided with two oppositely situated slots A leaving free passage to the electrodes and a tubular extension A" at its rear portion in which a -shaped tube D is fitted, the side portion of which serves for connection with the compressed air source while its straight rear portion is used as inspection tube for controlling the position of the electrodes.

The two side faces of the sheet metal casing C are arranged at 90 to each other and at with respect to the front face and carry, with the interposition of porcelain insulating plates E, metal tubes F serving as guide for the electrode and carrying a terminal I for the intake of the current. In order to obtain a correctly centered position of'the electrodes, the edge e of the porcelain plate is ground and rectified to a right angle with respect to the bore of the guiding tube F, which is turned inside after having been fitted by means of cement and a locking threaded ring f" on the said plate.

A cylindrical cap G provided with a hinged 5 front door G protects the electrode and other high voltage parts from external contacts.

A flywheel screw H is fitted centrally on the door and with the interposition of an insulator it carries a pin adapted to control the position of the electrode, which is pressed backwards against it by a spring h". Two small connecting pieces a g are also secured to the cylindrical cap for the inlet and outlet of the cooling water, which is conveyed to and discharged from the electrode through a double glass coil 9" g" of small section and considerable length to ensure a suitable electric resistance.

The electrode is constituted by two copper tubes M and m, arranged one within the other and serving the former for the inlet and the latter for the outlet of the cooling water; the tube M is provided with two guiding collars M effecting at the same time a tight closure so that the air blown into the chamber A of refractory material does not escape through the slots A, so that an airpressure equal to that existing in the chamber A is formed in the casing C, even if the pressure in the cap G corresponds only to the outer pressure.

The electrode carries at its front part a 1101- low head P of stainless steel having a threaded hole at its rear part for connection to the copper tube M. The electrode carries-at its rear portion a plate 'M" on one face of which bears the pin of the screw H and on the other face the spring 11."; the tube M and m end in nipples g' and g, respectively connected by rubber hoses to the glass coils for conveying the cool ing water. Spring tongues M' on the intermediate portion of the electrode ensure the contact of the electrode with the tube F and allow of an easy replacement of the electrode.

In the example shown'on Figs. 2 and'3 the above described heating system is shown applied to a small muiiie kiln R for baking pottery. The system is mounted on a box S enclosing the feeding unit, with the operating and measuring members and carrying at its rear portion a fan T for the blown air. The box S is mounted on wheels so that the heater can be easily displaced for inspection, rechange of parts and the like.

To start working it will be sufficient to connect the transformer to the ordinary voltage electric line and the coil to the water supply.

Fig. 4 shows the application of the heating system to a crucible furnace U for melting high melting metals; In this case the compressed air is drawn from a central tank through a conduit V. v

For large furnaces a small number of heaters of big size or a great number of heaters of small size may be used. In the case of big sized heaters the chamber in which the arc strikes should be wider than otherwise required in order that the small quantity of air, which is necessary for' transferring the heat, may have the required speed for blowing the arc. The air jet is confined to the central part of the chamber and thrown through a narorw slit thus forming an the discharged hot air. Such recovery however may be convenient for furnaces of large size when provision to this effect is not yet made by means of water heaters or other apparatus or systems utilising the hot air issuing from the furnace. In this case it will be posible to design a system for drawing the discharge air which is still in a hot condition and blow it upon the arc; for this purpose a fan is used having a flap of refractory material, a water cooled spindle and I a nut also coated with refractory material and well insulated.

What I claim is:

1. An electric furnace comprising a hollow refractory member having an extended outwardly tapering body section and a hollow depending relatively narrow section joining the tapering section near its neck region, said member having slots through its neck region, a plurality of electrodes arranged about said refractory member having head portions extending through said slots and forming a gaseous passage between said depending and tapering member sections, means for supplying electric current to support a substantial are within said refractory member between said electrode heads substantially at said neck region, gas pressure means connected to said depending section for blowing a Jet of gas between said electrode heads for forcing the are into said outwardly tapering refractory portion substantially away from said heads and transfer the generated heat to a distance therefrom.

2. An electric furnace comprising a hollow refractory member having a flared front section and a rear section of reduced diameter, said member being provided with opposed slots located between said sections, an enclosure surrounding said rear section, electrodes adjustably mounted within said enclosure and having heads reaching within said hollow refractory member through said slots and forming an air passage between said sections, means operable from the outside of said enclosure for adjusting the size of said air passage, means for supplying electric current to support a substantial are within said refractory.

member between said electrode heads, and air pressure means connected to said rear section for blowing a jet of air between said electrode heads forcing said are into said flared front refractory section substantially away from said electrode heads and transfer the generated heat to a distance therefrom.

3. An electric furnace comprising a tubular member of refractory material having a front section; with its internal diameter increasing towards the outlet and a rear section of reduced inner diameter, said tubular member having opposed slots located between said sections, a flange extending from said tubular member between said slots and said front section, a box tightly secured to srid flange and enclosing the rear end of said rear tubular member section, electrodes adjustably mounted in said box and each having a head reaching within said tubular body through said slots, means .on said box operable from the outside for adjusting the distance between said heads, and means within said box for supplying air under pressure to the rear portion of said tubular member for blowing the arc towards the outlet end of said front section of said tubular member, said last mentioned means being provided with an opening to permit observance of the electric are through said rear section.

4. An electric furnace, comprising a tubular member of refractory material having at an intermediate point two diametrically opposed slots, two flanges integral with said member on opposite sides of said slots, a metallic box tightly secured between said flanges and having two apertured walls substantially at 90 degrees with respect to each other, a'cap tightly fitted to each'oi said walls, a sleeve of refractory material clamped in the opening in each of said walls between said cap and the peripheral edge of said opening, a tube of conducting material tightly secured in each of said sleeves and provided with current feeding means, an electrode extending through each of said sleeves comprising head and shank portions, each shank having at an intermediate point two guide collars sliding in air-tight and in electrical contact relation with an associated one of said conducting tubes, a cover for closing each of said caps, the heads of said electrodes .being arranged to project within said member through an associated one of said slots, means for adjusting the distance between the electrode heads, and means within said box and between said caps for supplying a stream of air under pressure into the tubular refractory member through the electrodes for blowing an arc between them towards the outlet of said tubular member.

5. An electric furnace comprising a conical tubular member of refractory material provided with diametrically opposed slots at its narrowest portion, a coaxial tubular refractory section integral with said member and extending behind said slots thereof, a refractory attachment flange integral with said member on the side of saidslots remote from said extending section, and a collar integral with said extending section on the other side of said slots, a metallic box tightly secured between said flange and said collar and having two apertured walls substantially at 90 degrees to each other, a cap tightly fitted on each of said apertured portions, a sleeve of reiractory material tightlyclamped in the aperture in each of said walls, a guide .tube tightly secured in each of said sleeves and adapted to receive electric current, electrodes mounted for air-tight displacement and in electrical contact with said tubes and having a head reaching within the hollow of said member through the slots therein,

resilient means for mechanically biasing said electrodes outwardly, closing covers for said caps, means on said covers for adjusting said electrode heads against the action of said resilient means, water cooling means for said electrodes comprising glass coils of suflicient length preventing current dispersions, and means in said box for conveying a stream of air under pressure through said extension section and between the heads of said electrodes for blowing an arc between them towards the outlet of said tubular member, said last mentioned means being provided with an opening for observing the adjusted distance between said electrode heads.

' GIUSEPPE NIGRA. 

